This invention relates to a demodulator circuit and, in particular, to a synchronous-detection demodulator circuit which is widely used in various communication systems, for example, in a mobile communication system.
In the mobile communication system, it is necessary to use a modulation and demodulation device (modem) for carrying out burst operation. This is because communication is frequently interrupted in the mobile communication system. The modem comprises modulator and demodulator circuits which will be described in the following.
The modulator circuit is for modulating an original analog signal by phase-modulating a plurality of phases of a particular number into a modulated signal having a particular frequency. The modulated signal is used as a carrier in the mobile communication system in the manner known in the art.
The demodulator circuit is for demodulating the modulated signal into a demodulated signal. The demodulated signal is a reproduction of the orginal analog signal.
In the manner which will later be described, a conventional demodulator circuit comprises a signal converting circuit, a frequency multiplier, a signal processing circuit, a frequency demultiplier, and a signal producing circuit.
The signal converting circuit is for converting the modulated signal into a digital signal. The frequency multiplier is for multiplying the particular frequency by the particular number to produce a multiplied signal. The signal processing circuit is for processing the multiplied signal into a modified signal of a multiplied frequency. The frequency demultiplier is for demultiplying the multiplied frequency by the particular number to produce a demultiplied signal of the particular frequency. The signal producing circuit is for producing the demodulated signal in accordance with the digital and the demultiplied signals.
The conventional demodulator circuit has a disadvantage in that it is difficult to continuously and accurately carry out demodulating operation as will later be discussed in detail.